Conventionally, metallic moldings used in automobiles, electric and electronic appliances, and business and office machines have been replaced with synthetic resin moldings, with a view to lightening, energy-saving, reduction of prices, and the like. Synthetic resins used for the above purposes are resins having good heat resistance and high-impact properties, for example, a polymer alloy of polycarbonate and ABS (acrylonitrile-butadiene-styrene), or modified PPE (polyphenylene ether).
On the other hand, an AS (acrylonitrile-styrene) resin is superior in chemical resistance and mechanical strength, is transparent, and has excellent compatibility with the ABS resin or the like. Therefore, the AS resin is widely used as a molding material. However, the AS resin has a defect that it is inferior in heat resistance and hence is unsuitable for the use under a high temperature. Therefore, to enhance the heat resistance of the AS resin, for example, a terpolymer using N-substituted maleimide, that is, acrylonitrile-styrene-N-substituted-maleimide, is proposed.
However, acrylonitrile is not as reactive as styrene or N-substituted maleimide. Therefore, in the case where acrylonitrile, styrene, and N-substituted maleimide are mixed at a single step so as to be copolymerized, ratios of styrene and N-substituted maleimide in a structural unit of the thermoplastic copolymer in a first half stage of the copolymerizing reaction are high, whereas a ratio of acrylonitrile in the structural unit of the thermoplastic copolymer in a latter half stage of the copolymerizing reaction is high. In other words, the composition of the thermoplastic copolymer differ in the first half stage and in the latter half stage during the copolymerizing reaction. As a result, the thermoplastic copolymer obtained has a non-uniform composition. Therefore, the thermoplastic copolymer thus obtained is inferior in heat resistance, mechanical strength, and transparency.
Then, various manufacturing methods for manufacturing a thermoplastic copolymer having a uniform composition have been proposed. For example, the Japanese Publication for Laid-open Patent Application No.3-205411/1991 (Tokukaihei No.3-205411) discloses a method wherein first, all of acrylonitrile, all of N-substituted maleimide, and a part of styrene are mixed at a time and are continuously polymerized in a solution, then, the other part of styrene is added thereto, and thereafter the mixture is let mature. The Japanese Publication for Laid-open Patent Application No.7-278232/1995 (Tokukaihei No.7-278232) discloses a method wherein emulsion polymerization is carried out by dropping an emulsion prepared by emulsifying acrylonitrile, styrene, and N-substituted maleimide. As a method for obtaining a thermoplastic copolymer superior in mechanical strength, the Japanese Publication for Laid-open Patent Application No.63-162708/1988 (Tokukaisho No.63-162708) discloses a method wherein solution polymerization is carried out by first mixing acrylonitrile and styrene at a single step and thereafter adding dropwise the N-substituted maleimide thereto.
However, satisfactory uniformity of the composition cannot be achieved by the above-described conventional methods, and hence the thermoplastic copolymer at the latter half stage of the copolymerizing reaction has a higher ratio of acrylonitrile in its structural unit. Besides, usually a thermoplastic copolymer having a higher ratio of acrylonitrile in a structural unit is easy to discolor when heated. The thermoplastic copolymer has a great difference between a degree of discoloration before the heating and that after the heating (has a great degree of re-discoloration). In other words, the thermoplastic copolymers obtained by the conventional methods have a problem that they are easy to discolor and the re-discoloration while being processed in molding cannot be restrained. Therefore, a thermoplastic copolymer which is superior in heat resistance, mechanical strength, and other physical properties and is hard to discolor is earnestly sought for, along with a method for manufacturing the same.
The present invention was made in light with the above-described problems, and the object of the present invention is (1) to provide a thermoplastic copolymer which is superior in heat resistance, mechanical strength, and other physical properties, and is hard to discolor, and (2) to provide a method for manufacturing the same.